The present invention is related in general to devices used in opto-electronic applications, and, in particular, to a loss-of-signal (LOS) detector for optical signals.
Optical communication systems typically include optical receivers to receive an incoming optical signal from a channel, and convert the incoming optical signal into a current signal, which is amplified by electronic amplifiers. One way of measuring the performance of such an optical communication system is to measure the true loss of signal (LOS) event in the channel. A typical optical receiver may include an LOS detector.
A method of measuring a true LOS event is described in U.S. Pat. No. 5,563,893 to Lai (xe2x80x9cLaixe2x80x9d). Lai teaches a digital LOS detector having a trough circuit and a smearing circuit. The trough circuit generates a logic output whenever an incoming communication signal is within a predefined voltage range. The output of the trough circuit is applied to the smearing circuit, which generates an LOS output whenever the output of the trough circuit remains at a predefined logic level for a predetermined length of time. In a second embodiment, a state machine is coupled to the output of the smearing circuit, the trough circuit, and a clock, the state machine generating the LOS indication if the output from the smearing circuit remains at the predetermined logic level for at least a predetermined number of clock cycles.
In general, an optical communication system uses an Optical Supervisory Channel (OSC) to carry monitoring and control information. The OSCxe2x80x94which in the case of Dense Wavelength Division Multiplexing (DWDM) Synchronous Optical Network (SONET) is selected to have a wavelength of 1310 nm, 1480 nm, 1510 nm, or between 1610 and 1620 nmxe2x80x94is usually a separate channel from the data channels. Some optical amplifiers do not amplify signals at certain wavelengths such as those typically assigned to the OSC at 1510 nm, thereby resulting in loss-limited channels. In cases where the signal level is low, the known systems do not detect a true LOS event consistently and accurately. Accordingly, there is a need for an improvement in the art.
The present disclosure is directed to a method that advantageously uses certain observed patterns in amplified noise signals to determine an LOS condition in a loss-limited channel at an OSC wavelength or other wavelengths of interest. In one aspect, the disclosed method includes generating a sampling window of suitable size and counting the number of noise signal transitions that occur during the sampling window. If no transitions occur in a sampling window, then true data is assumed to be present during the sampling window. On the other hand, if transitions occur during the sampling window at a predetermined frequency, an LOS event is deemed to have occurred and an appropriate signal is generated. In another aspect, the transitions are counted and filtered, and if transitions exceed a predetermined number, an LOS condition is indicated.
In another aspect, the disclosed method uses a Clock and Data Recovery (CDR) circuit and adds simple digital logic circuits to detect an LOS condition in a predetermined sampling window. In a yet another aspect, the disclosure is directed toward a digital circuit including a window-detector circuit and a CDR circuit, which are coupled to an additional circuit to provide a sampling window. The window-detector circuit outputs a signal indicating an LOS condition if a predetermined number of transitions occur during a sampling window. In a further aspect, any LOS signal output by a conventional CDR circuit is logically OR""ed with the output of the window-detector circuit so that an LOS event is detected in either situationxe2x80x94where a conventional low-gain optical receiver is used, or where a full swing transitions may not be present with sufficient frequency to detect an LOS event.